Charging people to make a specific search engine the default for iOS search is arguably abusive of market power, but at least one could argue that it’s well-understood by consumers and competitors alike. Using that market power to demand a cut of revenue from private apps and then keeping it secret looks much worse.
A "potential competitor" that has no product at all? That sounds like nonsense.
They're paying for access, not to keep Apple out of search -- otherwise they'd pay a few extra billions on top so that users on Windows can't use that potential Apple search.
Apple has a search bot, that, from how it interacts with the sites according to my logs, is a decade or so behind. I'm sure they could change that if they bothered, but I doubt they will.
Why isn't Google paying Microsoft billions per quarter to stop Bing? Because Bing isn't a threat, and neither would Apple Search be. What is a threat is Apple making it hard for users on iOS to use Google Search. That's what the money is for, it's the rent they pay to have their little shop in the walled garden.
Entering a search term into the Safari URL bar will often pop up a webpage suggestion. It isn’t the first Google result. (Totally random example: try “Apache Spark tutorial.” Safari suggests a Databricks article, Google suggests something from Tutorialspoint.
If it's quid-pro-quo, ala Firefox funding itself via search defaults, I don't see how it's exploitative. I highly doubt Google is paying just to keep themselves in the Apple app store. They would (and could) have made a big fus about it unless they got something in return from Apple.
What are they getting from Apple, then? That’s the question that raises my anti-competitive fears. Presumably if Apple and Google wanted that question to be easily-answered they wouldn’t have written a secret deal.
Every answer to this question raises more questions not answered by the T-shirt. When two enormously powerful organizations are secretly cutting deals, the chance that somebody’s losing is very high.